Background and problem

The massive exploitation of crude oil in the past decades has remarkably reduced the number of easily accessible oilfields around the globe. Despite the possibility to promote a transition towards greener sources of energy, the upcoming shortage of cheap oil will also have a negative impact on the chemical industry. Most carbon-based platform chemicals, such as aromatic monomers for plastic production, are indeed directly obtained from crude oil fractions. For this reason, a lot of effort has been made in the past few years to develop a complete lignin-first biorefinery, to obtain aromatics and other chemicals from renewable biomasses [1]. Together with cellulose and hemicellulose, lignin is one of the main polymeric components of the plant cell walls. In the paper production process, it is removed with aggressive depolymerization techniques and used as low value fuel. This is however a waste of chemical potential, as lignin has the highest content by weight of aromatic units among natural polymers. In the lignin-first approach, on the other hand, lignin is looked at as a valuable component of the biomass. New catalysts and innovative processes have been developed to depolymerize lignin with high yield in a mixture of few phenolic monomers, among which (alkyl)guaiacols, leaving the cellulosic fraction almost unaffected for further usage [2].

Zeolites are playing a key role in the valorization of lignin-derived compounds, thanks to their ability of withstanding harsh reaction conditions, their micropores-induced selectivity and all the general advantages related to heterogenous catalysts. However, many reactions in lignin conversion would be ideally conducted in hot liquid water (HLW), an environment in which zeolites are known to be generally unstable. This is the case, for instance, of the demethylation reaction conducted on lignin-derived (alkyl)guaiacols to obtain catechol, which can further be used as building block to produce plastics and fine chemicals [3]. While some frameworks are known to withstand the harsh HLW conditions, it is now well established that a (partial) hydrolysis of some bonds is inevitable. Specifically, the Al-O bonds of the zeolite active site are the most susceptible to hydrolysis, leading to the formation of (partially) hydrolyzed Al species [4]. How these species affect the catalytic process, possibly completely changing the reaction mechanism and intermediates with respect to “traditional” Brønsted acid sites, is so far not known. Being these microscopic effects impossible to characterize experimentally but critical to understand and optimize the reaction conditions, a theoretical investigation is going to be performed to elucidate the still unknown effects on the (partially) hydrolyzed Al sites on guaiacol demethylation when HLW is present in zeolites (Figure 1).

Goal

The focus of this master thesis is a thorough understanding of the water-induced defects role on demethylation reactions related to biomass conversion in the H-BETA zeolite, which has already been shown to be suitable as catalyst for the process. Considering the harsh conditions at which this reaction is performed (liquid water, T>200°C, p>50bar) and the mobility of water molecules, the investigation will be performed with state-of-the-art molecular dynamics simulations. You will start by investigating the behavior and stability of various defective sites (regular Brønsted acid sites, water-induced open sites in the framework and extraframework aluminum species) while hot pressurized water is present in the zeolite pores, possibly comparing the behaviour of enclosed water with the pure liquid phase. Subsequently, guaiacol is going to be introduced in the zeolite and its influence on the water behavior as well as its interactions with the active site elucidated.

You will look for one or more possible dealkylation mechanisms and investigate them with advanced sampling techniques. This will allow you to accurately quantify the differences among the various possible defects, in terms of reaction barriers and reaction mechanisms, thus obtaining for the first time a complete overview on how they affect the reactivity of environmentally relevant molecules in zeolites and water.

The Center for Molecular Modeling (CMM) has a large experience in the study of zeolite-catalysed reactions with advanced molecular simulations. You will be coached and guided when needed to become acquainted with the software and techniques required to study the systems of interest. The computational power necessary for the calculations will be ensured by CMM’s access to the main national supercomputing infrastructures. Being the proposed research topic extremely relevant in the current chemistry research, the experimental investigations of such reactions is currently ongoing. You will then also have contact with the experimental group of Prof. Bert Sels (KU Leuven) and Prof. Bert Maes (UAntwerp), with whom the CMM has a consolidated collaboration.